Many forms of sensor have been proposed, and commonly these rely on some form of membrane to control the extent to which the components present in a sample under examination can gain access to an electrode, at which they can then be detected and determined. Especially, it well known to make sensors using membranes to separate the media being analysed from the active electrode itself. The main function of the membrane is to separate, as far as possible, those components which are desirable (i.e. can take part in the reactions at the electrode on which the desired determination depends) from interferents (i.e. compounds which may be present but are undesirable because they either interfere with the progress of the desired determination reactions or take part in reactions of their own which compete with those of the component sought and distort or overwhelm the signals which are to be measured. The forms of construction have much in common with each other, and mainly differ in the nature of the membrane or media within it or combined with it in some way.
Some forms of sensor rely on the components used to make the membrane, while others rely on the mode of fabrication of the membrane, selecting its physical properties (for example its porosity) or treatments given to it, as these factors can control its effectiveness and selectivity in use, or to the conditions under which the sensor is used.
Other forms of sensors incorporate an enzyme, which converts one substrate compound or analyte into another which may then be more easily measured. Especially, it is known to use oxidase enzymes, which generate hydrogen peroxide--a substance which can be measured very conveniently and very accurately by electrolytic methods, especially amperometrically.
An example is European Patent No. 216577 (of ICI PLC) which specifies an enzyme electrode sensor with membrane barrier of low (&gt;5%) porosity.
Materials which have been proposed for the fabrication of membranes for sensors include polyvinyl chloride (PVC)--but it has been stated to be essential for it to be only in plasticized form, as described in European Patent Application No. 92302131.5.
It has also in proposed in European Patent Application No. 86308918.1 (Publication No. 225094) to use a sulphonated or unsulphonated polyaryl sulphone as membrane for a sensor of the enzyme electrode type.
The known sensors, utilising various membrane materials, are very valuable in analytical techniques and the analysis of biological fluids (e.g. blood) for the presence of substances considered critical for medical reasons--e.g. glucose and other materials which may be oxidisable or reducable, or have toxic properties. However, they are not entirely satisfactory in use, as membranes which are permeable to glucose are usually also permeable to other compounds which interfere with the analytical determination of the glucose. Membrane materials vary in the degree of permeability or selectivity towards various species which may be present in samples to be examined, so no membrane material is universally appropriate and the choice depends partly upon the particular application for which it is to be used.
It is common to require a sensor to be highly selective or, at least, to be very efficient at excluding compounds which can interfere with the accuracy of the detection and determination of a desired chemical species, for example at an electrode surface. It is also desirable to have a form of sensor which can be used with accuracy at higher concentrations of glucose than are practicable with the conventional sensors and study media.